There is a risk in aircraft fuel systems of a build up of static electricity on one component in comparison to another component to which it is connected. In the prior art, a so-called bonding lead is connected to each side of the joint in order to electrically connect the components and prevent static build up. An example of such a bonding lead is described in the prior art section of US2008/0078880.
Bonding leads are traditionally made of highly conductive materials in different forms. These satisfy equipotential and electrostatic discharge requirements but allow the flow of lightning current across the joint. There is a risk that a break in such a bonding lead can result in intermittent contact and a source of sparking should high levels of electricity flow in the lead.
An example of this problem is shown in FIGS. 8 and 9. A bonding lead 42 couples a pair of fuel pipes 40, 41 across a joint between the pipes. The pipes are fitted within a fuel tank, close to a fuel tank wall 43. If the lead 42 breaks, the resulting two parts 42a, 42b will hang down as shown in FIG. 9 and there is a risk that one of the parts (in this case 42b) will make intermittent contact with the fuel tank wall 43.
One solution to this problem, described in the applicant's earlier application XA2907/XA2500 (GB patent application no. GB0904007.2) is to form the bonding lead from a material with a relatively low conductivity. This results in a relatively low flow of current which reduces the risk of sparking in the event of a lightning strike. However this solution requires the electrical conductivity of the material forming the bonding lead to be changed depending on the overall length of the bonding lead. This makes it difficult to provide bonding leads in different lengths which nevertheless lie within the desired resistance range.